1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Prior Art
A power transmitting system for a motor vehicle generally uses a hydraulically operated clutch which is controlled by a hydraulic control apparatus as disclosed in laid-open publication No. 1-188756 of unexamined Japanese Patent Application No. 63-9188. This hydraulic control apparatus is adapted to operate the clutch between its engaged position and its released or disengaged position, by feeding a working fluid into one of the engaging and releasing chambers while allowing the fluid to be discharged from the other chamber.
The known hydraulic control apparatus indicated above uses a clutch control valve which establishes the engaging position of the hydraulic clutch by feeding the working fluid from a hydraulic power source into the engaging chamber of the clutch while discharging the fluid from the releasing chamber into a drain line, and which establishes the disengaged position of the clutch by feeding the fluid from the hydraulic power source into the releasing chamber while discharging the fluid from the engaging chamber into the drain line through an oil cooler used for the hydraulic system. The hydraulic control apparatus also uses a rapid release valve for rapidly releasing the hydraulic clutch such that the oil cooler is bypassed when the fluid discharged from the releasing chamber is drained.
In the hydraulic control apparatus for the hydraulic clutch as described above, the clutch may be operated erroneously without following control signals applied to the clutch control valve or rapid release valve, if any trouble such as sticking of a spool of the valve happens. For instance, the clutch may be engaged in spite of the control signal commanding the control valve to release the clutch. In another case, the clutch may be rapidly released without a rapid release signal applied to the rapid release valve. In these cases, the power transmitting system is not normally operated so as to assure smooth running of the vehicle. If the hydraulic clutch is brought to the engaged position without the control signal for engagement of the clutch, the engine of the vehicle may stall, or the stopped vehicle cannot be smoothly started. If the clutch is rapidly released without the control signal for rapid release of the clutch, the operating temperature of the working fluid may rise to an abnormally high level, and the fluid may leak from the hydraulic system due to expansion of the fluid, since the discharged fluid is not cooled by the oil cooler.